Modern aircraft must be capable of running over, taking off from, and landing on grounds which are in poor condition and which have large bumps or ridges of non-negligible height. A shock absorber used under such conditions must enable such obstacles to be passed over with little effort and without any danger of bottoming.
It is initially recalled that a conventionally designed shock absorber has two internal functions: a resilient suspension function provided by compressing a volume of air; and an energy absorption function provided by forcing an incompressible fluid through a throttling orifice. In the description below, the term "shock absorber" is concerned solely with the resilient function.
A prior art shock absorber of the type having a single chamber of compressible air provides a resilient function as shown in FIG. 1 which is a plot showing the compression stroke C of the shock absorber as a function of the force F applied thereto. The force corresponding to the average static load Cs of the aircraft is located in the steeply sloping portion of the curve (high stiffness) in order to provide the aircraft with adequate stability. Further, in order to avoid aircraft bouncing on landing, the shock absorber must have a bottom threshold S.
As a result there is only a small margin of residual stroke d between the force under average static load Cs and the force Ft which corresponds to the shock absorber being pushed fully home.
There also exist prior art shock absorbers having two chambers of compressed air, and these are generally referred to as "two-chamber" shock absorbers. They provide a resilient function of the kind shown in FIG. 2. When the shock absorber is subjected to a force, it begins by compressing a so-called "low pressure" volume of air (curve A) up to a certain pressure threshold Fe. Thereafter, beyond this threshold, a so-called "high pressure" volume of air is compressed (curve B). The average static load Cs is located either on the stiff portion of the curve A for the abovementioned reasons, thereby enabling an increased amount of reserve stroke to be obtained for a given load force compared with a one-chamber shock absorber, or else on the low-slope portion of the curve B in order to reduce the forces and giving a reserve amount of stroke d". However, in particular in the second above-mentioned case, such an amount of reserve stroke is inadequate for passing over large-sized obstacles at high speed.
The main aim of the invention is therefore to provide a shock absorber having a resilient function which is suitable for passing over large-sized obstacles while nevertheless minimizing the forces generated by passing over such obstacles. Another aim of the invention is also to provide a shock absorber having a low threshold and high stiffness at the end of the retraction stroke in order to obtain good stability.